
When my girl was scraped, they didnt find active mites either. Most skin problems, have scrapings done AFTER the damage is already done. The mites live, cause the damage, and then die off, so by the time a scrapping is done, there is no real evidence for the mites (from what I was told). My breed is prone to Demodex, so they treated based on what the skin presented like, and hoped for the best.
Demodex is something every pet is born with. It is passed on from the mother, to the pup during birth. If the pup (or adult dog as well, but more commonly seen in pups) is stressed out from something (a surgery, a move, etc) then these mites come out of dormancy and start eating the hair folicles. There sometimes is itching, but my girl didnt itch to much from it. Only at certain times.
Here is a link, to Veterinary Partner's article on Demodex. Its a good site, although sometimes confusing when you use the search menus, if you dont know exactly what you are looking for, and havent spelt it correctly.
http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=630The meds that I put my girl on where called "Advantage Multi". Dont know if you can get it outside North America though, but its a multi med, that covers heartworms, fleas, ticks, parasites, and demodex. Some other people I know, have used a bath called Imaverol. I didnt want to do this, as I wondered if I was putting my dog in something, but I had to protect my hands and arms with gloves, whether it was really safe for the dog.
Here is another link:
http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=1561Its a condition called Seborrhea, and it also presents at the places you have mentioned. Unfortunately, with this one, there is no cure, only management.
Hope this helps.